This invention pertains to microlithography apparatus and methods as used, for example, in the manufacture of semiconductor integrated circuits and displays. More specifically, the invention pertains to such apparatus and methods that employ a charged particle beam (e.g., electron beam or ion beam) as an energy beam for performing projection-transfer of a pattern, defined by a segmented reticle, onto a sensitive substrate such as a semiconductor wafer. Yet more specifically, the invention pertains to such apparatus and methods exhibiting greater pattern-transfer accuracy whenever a segmented stencil reticle is used.
The known prior art is summarized below in the context of electron-beam microlithographic systems as representative charged-particle-beam (CPB) microlithographic systems. Whereas electron-beam microlithography potentially is more accurate for performing pattern transfer than optical microlithography (including optical microlithography performed using ultraviolet light), conventional experience with electron-beam microlithography has been plagued by, among various problems, low xe2x80x9cthroughputxe2x80x9d (number of wafers that can be exposed per unit time).
Various approaches have been investigated to increase throughput. One approach is xe2x80x9ccell projectionxe2x80x9d which is conventionally used whenever the pattern comprises a small basic unit portion (measuring, e.g., (5 xcexcm)2 on the wafer) that is repeated a large number of times in the pattern, such as a pattern for a memory chip in which the unit portion is a single memory cell. An image of the single unit portion is transferred to the wafer per exposure dose (xe2x80x9cshotxe2x80x9d); hence, many shots are required to transfer all the unit portions in the pattern. The same unit portion can be defined in multiple regions on the reticle. Unfortunately, circuit patterns such as memory chips include portions that are not repeated, and transfer of such portions requires application of another technique such as xe2x80x9cvariable-shaped beamxe2x80x9d lithographic writing. The need to use multiple techniques to achieve transfer of the entire pattern reduces throughput. In practice, the throughput achieved with cell projection is typically less than ten.
Another conventional approach (termed xe2x80x9cfull-field exposurexe2x80x9d), in which a reticle defining an entire pattern is transferred in one shot to a corresponding die on the wafer, offers prospects of very high throughput. Unfortunately, however, the very large exposure field required necessitates using electron optics having a correspondingly extremely large field. Such large electron-optical systems are prohibitively costly and bulky. Also, in such large fields, the peripheral regions of the field as projected tend to exhibit large aberrations that have been impossible to date to adequately correct. Furthermore, a reticle for use with full-field exposure is extremely difficult to fabricate.
In response to the problems with the full-field exposure technique, the xe2x80x9cdivided-pattern projection-exposurexe2x80x9d technique was proposed. In the divided-pattern technique, a reticle (mounted on a movable reticle stage) defines the entire pattern to be transferred to a corresponding die on the wafer (mounted on a movable wafer stage). Rather tan being exposed entirely in one shot, the pattern field as defined on the reticle is divided into multiple xe2x80x9cexposure unitsxe2x80x9d (e.g., xe2x80x9csubfieldsxe2x80x9d) that are individually and sequentially illuminated. Illumination is performed by an xe2x80x9cillumination beamxe2x80x9d passing through an xe2x80x9cillumination-optical systemxe2x80x9d located upstream of the reticle. An image of the illuminated exposure unit passes (as a xe2x80x9cpatterned beamxe2x80x9d) through a xe2x80x9cprojection-optical systemxe2x80x9d located between the reticle and the wafer. The projection-optical system has a field that is much smaller than the field of the entire pattern as defined on the reticle. The image that is projected by the projection-optical system onto a corresponding region of the wafer is xe2x80x9cdemagnifiedxe2x80x9d or xe2x80x9creduced,xe2x80x9d by which is meant that the image is smaller (usually by an integer xe2x80x9cdemagnification ratioxe2x80x9d such as 1/4 or 1/5) than the corresponding exposure unit on the reticle.
Systems that perform divided-pattern projection-exposure achieve lower throughput than the full-field exposure technique but substantially higher throughput than the cell projection technique. For details on divided-pattern projection-exposure, see, e.g., U.S. Pat. No. 5,260,151, incorporated herein by reference, and Japan Kokai Published Patent Document No. Hei 8-186070.
In divided-pattern projection-exposure, two basic types of reticles, termed xe2x80x9cstencilxe2x80x9d and xe2x80x9cmembranexe2x80x9d reticles, are currently used. Stencil reticles are usually configured as xe2x80x9cscattering-stencilxe2x80x9d reticles in which pattern features are defined by corresponding voids (openings) extending through the thickness dimension of a silicon membrane approximately 1 to 5 xcexcm thick. Charged particles in an illumination beam incident on an exposure unit of such a reticle pass through the voids without being scattered or absorbed by the reticle. In contrast, charged particles of the illumination beam incident on the membrane itself also pass through the membrane, but are scattered during such passage. To prevent such scattered particles from reaching the wafer, a xe2x80x9ccontrast aperturexe2x80x9d is situated in the projection-optical system at or near the conjugate plane of the entrance pupil of the projection-optical system (which is also the Fourier plane of the reticle surface). Particles that are not scattered pass through an axial aperture defined by the contrast aperture, whereas scattered particles are blocked (absorbed) by the contrast aperture and thus prevented from propagating to the wafer. Particles of the beam passing through the axial aperture are not further scattered and form an image of the illuminated exposure unit of the reticle on the wafer.
In a stencil reticle, the feature-defining voids are termed xe2x80x9cwhitexe2x80x9d regions and surrounding membrane regions are termed xe2x80x9cblackxe2x80x99 regions. The white and black regions collectively define the pattern defined by the reticle. Certain features defined by a stencil reticle include so-called xe2x80x9cislandxe2x80x9d regions that are defined by a black region surrounded by a white region. As readily can be surmised, an island (black) region cannot be situated within a surrounding white region in a stencil reticle because the island region would not have any physical support. Such a problem is referred to as the xe2x80x9cstencil problemxe2x80x9d or the xe2x80x9cdonut problem.xe2x80x9d
To solve the donut problem, the exposure unit containing an island region is divided into two xe2x80x9ccomplementaryxe2x80x9d exposure units in which the white region surrounding the island black region is divided in a manner providing (in each complementary exposure unit) physical support for the island black region. Each complementary exposure unit is individually exposed onto the same region on the wafer. Such double exposure on the same region of the wafer ideally results in the corresponding two images being in accurate registration with each other to form the complete island region. Unfortunately, the need to perform two exposures on at least some of the exposure units of the reticle correspondingly decreases throughput.
Another solution to the donut problem encountered with stencil reticles is to use instead a xe2x80x9cscattering-membrane reticlexe2x80x9d that is not subject to the donut problem. In a scattering-membrane reticle, a patterned layer of a high-scattering material is layered on a membrane made of a low-scattering material. The high-scattering material (e.g., chrome or tungsten approximately 10 to 200 nm thick) causes a high degree of scattering to particles of an illumination beam incident on an exposure unit of the reticle, even though such particles are transmitted by the membrane. The low-scattering material is typically a thin silicon membrane (approximately 100 nm thick) that transmits particles of an incident illumination beam while imparting relatively little scattering to the transmitted particles. Whenever a scattering-membrane reticle is used, highly scattered transmitted particles are blocked by a contrast aperture configured and situated as described above. An image of the illuminated exposure unit of the reticle is formed on the wafer by the transmitted (but little scattered) particles transmitted by the silicon membrane. By using a scattering-membrane reticle, throughput is improved compared to when using a stencil reticle because island regions can be transferred with a single shot.
Even though a scattering-membrane reticle requires only single exposures of each exposure unit of the reticle to transfer the pattern to the wafer, as discussed above, the extreme thinness of the membrane imposes substantial problems in reticle fabrication. On the other hand, even though double exposures of complementary exposure units are required with a scattering-stencil reticle to solve the donut problem, most of the membrane itself is much thicker and thus much stronger than the membrane used in a scattering-membrane reticle. This greater strength is a substantial advantage.
A typical area of an exposure unit as projected onto the wafer is one to several hundred micrometers square. The size of one die (chip) on the substrate can be 20 mmxc3x9740 mm. To produce a complete die, the images of individual exposure units on the wafer are xe2x80x9cstitchedxe2x80x9d together in a two-dimensional array by means of stage-position control and/or CPB deflection control.
To stitch together images of projected exposure units, the edges of adjacent images must be joined together at the xe2x80x9cseamsxe2x80x9d between the images. In conventional experience with stitching together such images, the images are frequently distorted along their edges, or the images are rotationally or positionally misaligned due to errors in stage movements and/or beam deflection. Such phenomena arise regardless of which type of segmented reticle is used, and adversely affect stitching accuracy. As a result, feature linewidth accuracy in the projected pattern is less than desired. In addition, if a scattering-stencil reticle is used, an image formed by exposure of two complementary exposure units on the same location on the wafer is prone to misalignment or misregistration of different portions of the same feature with each other. Misalignment and/or misregistration within an individual feature is usually caused by an error in stage position or rotation and/or an error in beam deflection from the first exposure to the second, and usually results in loss of control of linewidth of the feature.
An example conventional manner in which portions of a feature are defined in complementary exposure units of a stencil reticle and exposed on a wafer is shown in FIGS. 5(a)-5(c). FIG. 5(a) represents the desired pattern portion 101 to be exposed. In the figure, the hatched areas 107 and shaded areas 109 are respective feature portions defined by xe2x80x9cwhitexe2x80x9d regions of a reticle and that receive, on the wafer, an exposure dose above the imprinting threshold of the xe2x80x9cresistxe2x80x9d layer on the wafer. The dashed lines 113 in the figure indicate boundaries between adjacent exposure units 111 on the reticle and denote the respective locations of seams between images of adjacent exposure units as projected on the wafer. In this example, the same feature patterns repeated in the four exposure units 111 shown. Inside each exposure unit 111 is an island region 110 defined by a corresponding black region on the reticle. Each island region is locally surrounded by a white region.
On a membrane reticle, even though each island region 110 is surrounded by a white region, an image of each exposure unit 111 can be formed on the wafer using only one respective exposure. No dividing of the exposure units 111 on the reticle into complementary exposure units is required. However, in the case of a stencil reticle, formation of an image of each exposure unit 111 requires exposure of each of two respective complementary exposure units 111a (FIG. 5(b) and 111b (FIG. 5(c). Each exposure unit 111a defines a first portion 107 of the respective feature, and each exposure unit 111b defines a second portion 109 of the respective feature. Both portions 107, 109 are required to define the complete white region surrounding the island region 110. In FIG. 5(b), the exposure units 111a are defined on a first reticle 103, and in FIG. 5(c), the exposure units 111b are defined on a second reticle 105.
As can be seen in FIG. 5(a), feature lines cross the boundaries 113. For optimal pattern-transfer accuracy and resolution, formation of images of the exposure units 111a and 111b on the wafer requires accurate registration of each image of an exposure unit 111a with an image of a respective exposure unit 111b, and accurate alignment of images of adjacent exposure units with each other. Unfortunately, in conventional practice, adequate control of such parameters is elusive, resulting in imaging errors and loss of linewidth control, especially adjacent the seams 113.
This invention addresses certain of the problems of conventional apparatus and methods summarized above. Hence, an object of the invention is to provide charged-particle-beam (CPB) projection-exposure methods and apparatus exhibiting improved pattern-transfer accuracy, especially when performed using a segmented reticle.
According to a first aspect of the invention, methods are provided for performing CPB projection-exposure. General aspects of such a method include the steps of dividing a pattern, to be projection-exposed onto a sensitive substrate, into multiple exposure units each defining a respective portion of the pattern. The exposure units are sequentially illuminated using a charged-particle (CP) illumination beam to form a respective CP patterned beam. The patterned beam is projected onto a sensitive substrate to form images of the exposure units at respective locations on the substrate at which the images of the exposure units are stitched together to form an image of the pattern on the substrate. According to a representative embodiment, and with respect to any exposure unit defining a feature requiring two separate exposures to fully transfer the feature to the substrate, each of such exposure units is divided into first and second complementary exposure units each defining respective feature portions. Boundaries are defined around each first complementary exposure unit, and boundaries are defined around each second complementary exposure unit. The boundaries around the first complementary exposure units do not cross over the respective feature portions defined by the first complementary exposure units, and the boundaries around the second complementary exposure units do not cross over the respective feature portions defined by the second complementary exposure units. In other words, the boundaries around a first complementary exposure unit are laterally shifted relative to the boundaries around the corresponding second complementary exposure unit, with respect to the features defined by the particular pair of first and second complementary exposure units. As a result, the boundaries around the second complementary exposure unit are shifted relative to the boundaries around the corresponding first complementary exposure unit whenever the feature portions defined by the second complementary exposure unit are placed in registration with the feature portions defined by the corresponding first complementary exposure unit on the substrate. I.e., the first complementary exposure units and the second complementary exposure units are projection-exposed onto respective locations on the substrate such that, when projection-exposing a second complementary exposure unit on an image of a respective first complementary exposure unit, the boundaries around the second complementary exposure unit are shifted relative to the boundaries around the respective first complementary exposure unit.
The first and second complementary exposure units can be defined on the same reticle (e.g., in different respective regions of a single reticle) or on separate respective reticles.
According to another aspect of the invention, segmented reticles (e.g., stencil reticles) are provided for use in CPB microlithography. A representative embodiment of such a reticle comprises multiple exposure units each defining a respective portion of a pattern to be projection-exposed onto a sensitive substrate. At least one exposure unit defines a feature requiring two separate exposures to fully transfer the feature to the substrate. Such an exposure unit is divided into first and second complementary exposure units each defining respective feature portions. Each first complementary exposure unit is surrounded by respective boundaries, and each second complementary exposure unit is surrounded by respective boundaries. The boundaries around the first complementary exposure units do not cross over the respective feature portions defined by the first complementary exposure units, and the boundaries around the second complementary exposure units do not cross over the respective feature portions defined by the second complementary exposure units. This causes the boundaries around the second complementary exposure units to be shifted relative to the boundaries around the first complementary exposure units whenever the feature portions defined by the second complementary exposure unit are placed in registration with the feature portions defined by the respective first complementary exposure units.
According to another aspect of the invention, CPB microlithographic projection-exposure apparatus are provided. A representative embodiment of such an apparatus comprises a substrate stage on which a sensitive substrate is mounted for CPB projection-exposure of the substrate, and a reticle stage on which a reticle is mounted. The apparatus includes a reticle as generally summarized above. The apparatus also comprises an illumination-optical system situated upstream of the reticle stage. The illumination-optical system is configured to sequentially illuminate the exposure units of the reticle with a charged-particle (CP) illumination beam. The apparatus also includes a projection-optical system situated between the reticle stage and the substrate stage. The projection-optical system is configured to project, via a patterned beam propagating downstream of the reticle on the reticle stage, an image of the illuminated exposure unit onto a corresponding location on the sensitive substrate so as to stitch together the exposure-unit images and form an image of the pattern on the substrate.
Again, as noted above, the reticle can be a stencil reticle. The reticle can comprise a first reticle portion defining the first complementary exposure units and a second reticle portion defining the second complementary exposure units. Alternatively, the first and second reticle portions can be located on separate reticles. Whereas the invention is especially useful with stencil reticles, the invention also has utility when using membrane reticles such as scattering-membrane reticles.
Losses in accuracy of exposure linewidth control at seams of adjoining exposure units are avoided with this invention because feature portions in complementary exposure units are not formed at the boundaries of the respective complementary exposure units.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.